Fire alarms are characteristically robust electrical/electronic devices, expected to resist abuse and events such as installer error, while maintaining functionality for extended periods with minimal maintenance. Typical fire alarm apparatus may employ automated sensors as triggering devices, which can include detectors for such hazards as smoke, heat, toxic vapor, and combustion products, along with manually actuated alarm stations. In addition to signal inputs, a fire alarm system necessarily includes outputs. Outputs can include, depending on the requirements of an application, acoustical, visual, and/or other signals distributed throughout a facility, message transmitters directed to emergency responders, facility managers, and the like, actuators such as fire door auto-closure functions, security lock releases, manufacturing process shutdown command signals, and such other functions as may be dictated by the requirements of an individual application.
Fire alarm systems typically employ premises power sources—nominal 115 or 230 VAC at 50 or 60 Hz single-phase is widely available—for normal operation of entire systems. Higher-power sources and specialty supplies can be incorporated where needed or preferentially available, such as DC, three-phase AC, a non-standard supply voltage or frequency, or the like.
Fire alarm system internal electronics generally employ power having a configuration appropriate for the technology. Thus, earlier systems may have used, for example, relays powered directly from premises power to actuate notification appliances—light and/or sound generators—distributed throughout a facility, while many successor systems have adopted DC power at lower voltages for their less power-demanding but more effective equivalent appliances. With the adoption of intrinsically safe low voltage DC power to actuate high-efficiency strobe lights and horns, a need to employ licensed electricians to install and maintain system components has diminished, potentially reducing costs.
Designs for applying power to notification appliance circuits (NACs) and auxiliary (AUX) functions of fire alarm electronics using distribution wiring are subject to laws in many jurisdictions, as well as to regulations and conventions that may have the force of law. Marketing pressure may likewise dictate particular approaches to apparatus design, in consideration of compatibility between vendors, regional and worldwide standards, cost, and other issues. For example, any telephone “dialer” device is likely to use a dual tone, multi-frequency (DTMF) code sequence, as originally developed by Bell Laboratories and now adopted substantially universally, to initiate all calls.
New regulations are scheduled for imminent adoption, both within the U.S. and in jurisdictions such as Canada, for which harmonization with U.S. regulations is largely in force. These regulations stipulate fire alarm circuit performance attributes such as transient current draw during normal NAC operation and short circuit foldback current versus time for AUX operation. While multiple alternatives are possible to address these regulations, many obvious solutions to implement the regulations are undesirably costly or complex. Similarly, conventional, non-regulation based technical solutions, such as use of manually reset protective devices, may be undesirable in a competitive and convenience-oriented marketplace.
Accordingly, it is desirable to provide a method and apparatus for supplying actuation power to distributed fire alarm notification appliance circuits and auxiliary loads whereby high transient current survival, intelligent power foldback, and automatic foldback recovery coexist in an economically practical configuration.